The issue I’m seeing with any free plans of email providers is that they either require an identifying verification, or don’t allow the email to be used for services.
tutamail, for example, states that the free plan is not to be used to signup for services.
protonmail requires verification by email ‘to prove you are human’.
Thank you
From my experience it seems to require verification or payment if you try to create multiple accounts over a short period of time, or if you create an account over Tor or a VPN. There might be other triggers that set it off but those are the ones I’ve found